8051 is not dieing atmel still making 8051 chips and lots of other vender's are making it
NXP is one of them. I will suggest 8051 because Lots of good books available for start learning. also I think this is the easiest controller to start with. you don't need costly development tools. at89s52, P89V51RD2 can be programmed by self made $2 programmer
upto 64K memory to program. free and very good compilers and IDE are available like SDCC and MIDE simulation is not a problem.
AVR is fast and lots of goodies in built. but those who are starting with microcontroller 8051 is the answer not PIC and AVR. after learning 8051 for at least for 6 months they can switch depending there needs because now they can take decision.

You also have this one http://e2e.ti.com/group/msp430launchpad/w/default.aspx I am PIC person myself. But I think it offer great value for the money.
Atmel also have some series of controllers based on the 8051 instruction set. I do not think it matter much if you choose to start with a controller from Microchip or Atmel. However I think perhaps the PIC programmers are both cheaper and better. If you plan to purchase a programmer/debugger. You should ask for advice in this forum before purchasing. You will find a lot of stuff on ebay. Some is good, and some is far from good. What is your budget?